1. Field of the Invention
The present invention relates to an organic electroluminescence (EL) display device having a region emitting light toward a substrate and a region emitting light away from the substrate, and more particularly, to an organic EL display device having a first organic light emitting unit that emits light away from the substrate and a second organic light emitting unit that emits light toward the substrate.
2. Discussion of the Background
Generally, an organic EL display device is a self-emitting device that electrically excites a fluorescent organic compound to emit light. It is considered a next generation display device because it may be driven with a low voltage, it is thin, and it has a wide viewing angle and rapid response speed. In the organic EL display device, an organic layer may be formed on a transparent insulating substrate, such as glass, and between electrode layers. With such a structure, when applying positive and negative voltages to the electrode layers, respectively, holes and electrons travel to the light emitting layer. Holes, which may be transported though a hole transport layer, may be injected from the electrode to which the positive voltage is applied. Electrons may be transported through an electron transport layer to the light emitting layer from the electrode to which the negative voltage is applied. The electrons and holes combine to generate exitons in the light emitting layer, and when the exitons transition from an exited state to a ground state, fluorescent particles on the light emitting layer emit light, thereby displaying images.
An active matrix (AM) type organic EL display device includes at least two thin film transistors (TFTs) per pixel. One TFT may be used as a switching device that controls the pixel's operation, and the other as a driving device driving the pixel. The TFTs may include a semiconductor active layer having a channel region between a drain region and a source region, a gate insulating layer formed on the semiconductor active layer, a gate electrode formed on the gate insulating layer and above the channel region, and a drain electrode and a source electrode coupled to the drain region and the source region, respectively, through contact holes in an interlayer formed on the gate electrode.
FIG. 1 is a plan view showing a pixel of a conventional organic EL display device, and FIG. 2 is a cross-sectional view showing the pixel of FIG. 1 along line I-I.
As FIG. 1 shows, the organic EL display device may include a plurality of pixels. A pixel may be surrounded by two adjacent scan lines Scan, two adjacent data lines Data, and a power line Vdd. Each pixel may include a switching TFT TFTsw, a driving TFT TFTdr, a capacitor Cst, and an organic EL device OLED. A pixel may include more than two TFTs and more than one capacitor.
The switching TFT TFTsw is driven by a scanning signal from the scan line Scan, and it transmits a data signal from the data line Data. The driving TFT TFTdr determines an amount of current flowing into the OLED through the driving line Vdd by the data signal from the switching TFT TFTsw, that is, a voltage difference Vgs between the driving TFT's gate and source electrodes. The capacitor Cst stores the data signal for one frame.
Referring to FIG. 2, which is a cross-sectional view along line I-I of FIG. 1, a buffer layer 111 is formed on a glass substrate 110, and the TFT and the OLED are formed on the buffer layer 111.
The AM type organic EL display device may be fabricated as follows.
A semiconductor active layer 121 is disposed on the buffer layer 111. A SiO2 gate insulating layer 112 is disposed on the semiconductor active layer 121, and a gate electrode 122 is formed on a predetermined portion of the gate insulating layer 112 as a conductive layer of MoW or Al/Cu. The gate electrode 122 is connected to a gate line (not shown) for applying on/off signals. An interlayer dielectric 113 is formed on the gate electrode 122, and source/drain electrodes 123 are coupled to the source region and the drain region of the semiconductor active layer 121, respectively, through a contact hole. The power line Vdd may be formed when forming the source/drain electrodes 123. An SiO2 or SiNx passivation layer 114 is formed on the source/drain electrodes 123, and an overcoat layer 115, which may be formed of an organic material such as acryl, poly-imide, or BCB, is formed on the passivation layer 114.
A via hole 115a exposing a source or drain electrode 123 is formed in the passivation layer 114 and the over coat layer 115 through a photolithography or perforation process. Additionally, a first electrode layer 131, as an anode, is formed on the over coat layer 115, and is coupled to the source or drain electrode 123 exposed by the via hole 115a. An organic pixel defining layer 116 is formed on the first electrode layer 131, and an organic layer 132 is formed in a region defined by an opening portion 116a of the pixel defining layer 116. The organic layer 132 includes the light emitting layer. A second electrode layer 133, as a cathode electrode, is formed to cover the organic layer 132. The organic layer 132 emits light by receiving holes and electrons from the first electrode layer 131 and the second electrode layer 133.
When the first electrode layer 131 is transparent and the second electrode layer 133 is reflective, light emitted from the organic layer 132 proceeds toward the substrate 110. This type of organic EL display device is referred to as a back surface emitting type. On the contrary, when the first electrode layer 131 is reflective and the second electrode layer 133 is transparent, light emitted from the organic layer 132 proceeds away from the substrate 110. This type of organic EL display device is referred to as a front surface emitting type.
However, in order to fabricate an organic EL display device that emits light in both directions, two organic EL display devices may be stacked upon on another.
For example, as FIG. 3 shows, substrates of front surface emitting type devices may be combined to fabricate an organic EL display device that emits light in two directions. Alternatively, second electrode layers 133 of back surface emitting type devices may be disposed to face each other to fabricate the organic EL display device that emits light in two directions.
Korean Patent Publication No. 2003-0019015 discloses a folder type mobile communication terminal using an organic EL display device. This organic EL device, formed by stacking two EL devices, displays an image of a back surface when folded and displays an image of a front surface when unfolded.
However, since the above organic EL display device comprises two, stacked organic EL display devices, the device's thickness and cost increase correspondingly, and an additional driving device is required. Moreover, since the two organic EL devices overlap, it may not be easy to install a getter for protecting the organic layer 132.